According to current prison population figures, 5% equates to just over 4,200 inmates.

That is equivalent to the total number of women in English and Welsh prisons.

The proportion has risen in the year since 2011/12, when 4% of prisoners told HMIP they identified themselves as Gypsy, Romany Gypsy or Irish Traveller.

The HMIP paper said: "Even on the lowest estimates... it is clear that prisoners of Gypsy, Romany and Traveller backgrounds are significantly over-represented in the prison population.

"Our survey findings suggest that the proportion might be as high as 5% - the same proportion as women prisoners - and much higher than this in some establishments, particularly those holding children."

It said the reasons for the over-representation "lie outside the prison service", adding that "more needs to be done to understand and address this".

HMIP said there were "strikingly high" numbers of such inmates in some prisons, according to their surveys.

'Equal treatment'

In 2012/13, 12% of prisoners at HMP Elmley, Kent, 11% at HMP Gloucester and 10% at HMP Winchester told HMIP they identified themselves as being Gypsy, Romany or Traveller.

The proportion of Gypsies was higher in secure training centres (STCs). A total of 12% of inmates in those centres, which hold 12-18 year olds, said they were Gypsy, Romany or Traveller.

However in young offender institutions, the proportion was the same as for adult institutions, at 5%.

A Ministry of Justice spokeswoman said: "We are committed to ensuring fair and equal treatment for all prisoners, including those with protected characteristics as defined in the Equality Act 2010.

"As part of this, we recently launched a campaign to encourage and support Gypsy, Traveller and Roma prisoners to declare their heritage and address some of the sensitive issues affecting them and their communities.

"Since the start of this, we have seen a 50% increase in declarations."

According to 2011 Census data, 0.1% of the population of England and Wales - 58,000 people - identified themselves as being Gypsy or Irish Traveller.